7.23.
Apply Layer Mask

The Apply Layer Mask command merges the
layer mask with the current layer. The transparency information in the
layer mask is transferred to the alpha channel, that is created if it
doesn't exist, and the layer mask is removed. If the active layer does not
have a layer mask, the menu entry is insensitive and grayed out. See the
Layer Masks section for more
information.

7.23.1.
Activating the Command

You can access this command from the image menubar through
Layer → Mask → Apply Layer Mask,

or from the pop-up menu you get by right-clicking on the active
layer in the Layers Dialog.